Jofra Archer, Buttler's Rajasthan Royals team-mate, led the outrage on social media, while Shane Warne, an ambassador for the team, described it as a "disgraceful and low act".
England star Buttler had scored 69 runs from 43 balls as his Rajasthan side attempted to chase down Kings XI Punjab's 4-184.
Before Buttler's dismissal, the game was more notable for the return of Smith following his ban.
But there was support for Ashwin too.
As the umpire sent it upstairs to confirm Buttler was out of his crease, the crowd erupted at the realisation that they had just witnessed the IPL's first Mankad. The dismissal of Buttler proved key to Punjab winning the game and the England wicketkeeper-batsman refused to shake hands with Ashwin afterwards.
Buttler started backing up and Ashwin, after landing for his bowling action, stopped, hesitated and knocked the bails off at the non-striker's end.
There was a 40-run stand between Steve Smith, who was playing his first IPL match since 2017, and Sanju Samson.
Ashwin, the Punjab captain, defended himself after the match. According to the rules, there was, of course, nothing wrong in what Ashwin did but the moment proved to be a instant source of controversy with Matthew Hayden, on air at the moment, blasting Ashwin. Given the kind of dismissal involved, a heated exchange happened on the field between the bowler and the batsman. "All captains sign the IPL wall and agree to play in the spirit of the game".
Gayle had struck 13 fours and 17 sixes en route to the fastest ever Twenty20 century off only 30 balls - 154 out of his 175 runs came in boundaries. He is also only the second overseas player after David Warner (4,099) to reach the landmark.